1. Field of the Invention
The present invention relates to maintaining copies of data in a storage management system.
2. Background Art
Removable storage media, such as tape cartridges, are often used for long-term data storage. Often, these tape cartridges are held in locations remote from where they are written and read. Remote locations for storing data provide several benefits. First, remote locations tend to be less expensive per square foot than on-site locations. Second, remote locations better ensure that data can be recovered in the event of a catastrophic, on-site disaster. Third, copies of the data may be required for access at off-site locations.
The amount of data that must be stored, both on-site and off-site, continues to increase. One method for dealing with this increase is to use data cartridges that hold a greater amount of data in a given volume. Such data cartridges typically hold multiple data volumes or data sets. One example is multi-volume tape cartridges, each of which can hold one or more virtual tape volumes.
Storage management systems track multi-volume tape cartridges and map the assignment of virtual tape volumes to multi-volume cartridges. In a typical storage management system, two copies of data are written to removable storage media. This results in a given virtual tape volume being recorded on two separate multi-volume cartridges. One multi-volume cartridge is stored locally for archival and/or back-up purposes. The other multi-volume cartridge is sent to an off-site location. The storage management system is configured to record information pertaining to both multi-volume cartridges.
A major problem with such storage management systems is the inability to rapidly retrieve data if the virtual tape volume containing the data cannot be read from the locally-maintained multi-volume cartridge upon which the virtual tape volume has been written. This can occur if the local multi-volume cartridge has been lost, damaged, accidentally overwritten, misplaced, or the like. Even if the physical multi-volume cartridge is found and is in good condition, the data may still not be readable due to difficulties with the storage media including defects, localized damage, wear, aging, and the like. Thus, the only way to obtain the desired data is to recall the off-site multi-volume cartridge, a process which can result in considerable delay and may violate disaster recovery policies.
What is needed is management of data copies which permit a second local removable copy to be maintained by storage systems not inherently designed to handle more than two copies. This management should produce and maintain data copies in a reliable manner without requiring extensive modification of an existing storage management system.